1. Field of the Invention
The present invention relates to semiconductor memory devices and in particular to a fuse detection method for detecting a program state of fuses and a semiconductor memory device including a fuse detection circuit.
2. Description of the Related Art
In recent years, large-scale integration of semiconductor devices is achieved along with miniaturization of semiconductor elements. This trend is seen especially in the field of semiconductor memory devices. For example, dynamic random access memory (DRAM) products having a memory capacity of 1 Gigabit have been developed and are in actual use.
These semiconductor memory devices include a redundancy memory cell array area for salvage having an extra memory cell array arranged therein, in addition to a main memory cell array area having a normal memory cell array arranged therein. A redundancy circuit for replacing any defective memory cell found in the main memory cell array area with a redundancy memory cell is employed. With this redundancy circuit employed, the production yield of large-scale-integrated semiconductor memory devices is improved and the cost associated with such semiconductor memory devices is reduced. These redundancy circuits require program elements for storing, for example, the addresses of defective memory cells. For these program elements, breaking fuses where polysilicon or metal wiring is cut with a laser trimmer are mainly used.
The application of the redundancy circuit includes (1) replacement of malfunctioning memory cells, (2) replacement of property-defective memory cells, and (3) replacement of low-grade memory cells. The replacement of malfunctioning memory cells in (1) is to salvage memory cells whose basic operation is problematic. This type of salvage is referred to as LFT (Loose Function Test)-defective salvage. The replacement of property-defective memory cells in (2) is to salvage memory cells whose basic operation is normal but, for example, whose switching characteristic or retention property is problematic. This salvage is referred to as property-defective salvage. The replacement of low-grade memory cells in (3) is to replace low-grade memory cells. For this purpose, several grades are set according to the tolerance from a standard value for memory cells that satisfy the standard value. This replacement is referred to as grade salvage.
A practical semiconductor memory device includes redundancy circuits with about 1000 links. Of these 1000 links, in general, about 20 links are used for LFT-defective salvage, about 200 links are used for property-defective salvage, and about 600 links are used for grade salvage. Therefore, most redundancy circuits are used for grade salvage. In the case of grade salvage, salvage starts with low-grade memory cells and proceed towards higher-grade memory cells. When a grade which cannot be salvaged is reached after all links have been used up, the memory cells with the previous grade and lower are replaced. Instead of using up all links, defective memory cells are replaced with redundancy circuits so that the same grades are maintained as chips.
When defective memory cells are replaced with redundancy circuits, it is necessary to confirm the replacement rate as to whether programmed fuses are normally cut and the defective memory cells are replaced as programmed. If programmed fuses are not cut normally, undesired memory cells are replaced with redundancy circuits instead of replacing defective memory cells with redundancy circuits. This results in the chips being determined as detective cells. Therefore, confirmation of the replacement rate is very important. However, since confirmation of the replacement rate is carried out with redundancy circuits for LFT-effective salvage for the following reason, the accuracy of the replacement rate is very low. This is problematic in that effective confirmation of the replacement rate cannot be carried out.
For replacement of defective memory cells with redundancy circuits, replacement for LFT-defective salvage exhibits a reproducibility of 99%, whereas replacement for property-defective salvage exhibits a reproducibility of only 95%. This is because determination as to whether fuses are normally cut also depends on factors such as differences in individual testers and temperature conditions. For grade-salvage replacement, even the reproducibility is unknown. Since grade-salvage replacement is intended to replace low-grade memory cells, the characteristic value of each memory cell needs to be compared directly with a reference value. For this purpose, the characteristic values of memory cells need to be checked one at a time, which is practically impossible. In practice, no more than investigating the extent of characteristic distribution can be carried out. Therefore, confirmation of the replacement rate is carried out using redundancy circuits for LFT-defective salvage with superior reproducibility.
Normally, the number of fuses used for one link of redundancy circuits is 10 to 12, where half the fuses are cut on average. Therefore, 5000 fuses need to be cut in order to program redundancy circuits with 1000 links. Thus, a desired product can be manufactured only after all fuses are cut correctly. With a cutting failure rate of 0.02% (2E-4), the chance of producing a non-defective chip is 50%. To increase this chance to 90%, a failure rate another order of magnitude lower is required. Furthermore, to achieve a success rate of 99%, it is necessary to keep a failure rate of about (1E-6) or lower. From a viewpoint of the replacement rate for LFT-defective salvage, accuracy with a failure rate of only about (2E-2) can be confirmed since about 2% of links are used for LFT-defective salvage. For this reason, whether a chip is non-defective cannot be determined. In short, there is a problem that no methods are available for easily detecting a successful fuse-cutting rate without including the difference in individual testers.
A method of investigating the states of programmed fuses is disclosed in Japanese Unexamined Patent Application Publication No. 2004-296051 (JP-A 2004-296051). It can be confirmed whether fuses have been cut as programmed using the invention disclosed In JP-A 2004-296051. According to JP-A 2004-296051, the program state of each fuse used in redundancy circuits can be confirmed. The number of programmed (cut) fuses is counted and is then compared with the number of fuses to be cut based on data indicating which fuses are to be cut in each chip to confirm whether or not fuses have been cut as programmed. Hereinafter, the percentage of fuses being cut as programmed is referred to as the successful fuse-cutting rate, and the percentage of redundancy circuits being substituted as expected is referred to as the replacement rate.
FIG. 5 shows a redundancy control block 10 described in JP-A2004-296051. The redundancy control block 10 having control units corresponding to each of n redundancy circuits includes redundancy-circuit selection circuits 11, redundancy decoder circuits 12, and a decoder killer circuit 13. The redundancy-circuit selection circuit 11 is a circuit that identifies the n redundancy circuits for selection and output a selection signal SelK. The redundancy decoder circuit 12 includes an enable determination circuit 14, a plurality of address determination circuits 15, and a decoder output circuit 16. In this example, m address determination circuits 15 corresponding to address signals A1 to Am are provided per link.
In a normal operation mode, the enable determination circuit 14 determines whether an enable signal coincides with the program state of an enable fuse, and the plurality of address determination circuits 15 determine whether each bit of an address signal coincides with the program state of an address fuse, thus outputting an enable determination signal EnK and an address determination signal RAi. In a test mode, the enable determination circuit 14 and the plurality of address determination circuits 15 input a low level to the signals for which the program state of a fuse is to be investigated and input a high level to the rest of the signals. The program state of a fuse of the signals to be investigated is output to the determination signals EnK and RAi.
The determination signals EnK and RAi are input to the decoder output circuit 16. In the normal mode, a decoder output signal RedK indicating whether the corresponding redundancy circuit is used is output, and in the test mode the decoder output signal RedK indicating the program state of the corresponding fuse is output. Here, fuse signals (Fen, Fi) of programmed and cut fuses are assumed to be output as high-level signals. The decoder output signal RedK is input to the decoder killer circuit 13, which outputs a killer signal 5. In the test mode, the redundancy control block 10 inputs a low level to signals for which the program state of a fuse is investigated and inputs a high level to the rest of the signals. In this manner, the program state of the fuse of signals to be investigated is output to the determination signals EnK and RAi. Therefore, the programmed and cut fuses can be counted by counting the killer signals 5 output in the test mode.
However, the invention of this earlier application has a problem in that it takes a long measurement time. Counting the number of cut fuses can be achieved by testing each fuse in one memory cycle and reading out the test result indicating whether the fuse is programmed (cut). However, normal testers do not have means for counting only the cut fuses. Although counting can be carried out using a fail bitmap, a tester having a fail bitmap is very expensive and hence, is not mass-productive. For this reason, one fuse is tested in one test cycle, and the result is determined.
Although the memory cycle during test is about 100 ns, a test cycle of about 1 ms is required to determine pass/fail of data. If one fuse is tested in one test cycle, the test time is prolonged. A time of about 10 seconds is required to test 10000 fuses for one semiconductor chip. Furthermore, if 500 to 1000 chips are mounted on one wafer, about a two-hour measurement time is required per wafer. This means that it is practically impossible to monitor the fuse program accuracy through mass-production online measurement. Therefore, even with the invention in the earlier application, the replacement rate of redundancy circuits cannot be confirmed through mass-production online monitoring of the successful fuse-cutting rate.
Semiconductor memory devices use redundancy circuits to improve the production yield and reduce cost. Unfortunately, it is not possible to confirm whether fuses of redundancy circuits are normally cut and the redundancy circuits are substituted as programmed.